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The cooling fans failing to run when the engine needs them. The distinction from fans running constantly is which way the system has failed, and this is the damaging direction: no forced airflow means overheating in traffic while everything is fine at speed.

That speed dependence is the signature. Temperature climbing when stationary and falling on the move is airflow, not circulation — the reverse of a thermostat or pump fault. The causes run in order of likelihood: a blown fan fuse, a failed relay, a failed control module, a failed motor, or a temperature sensor reading low enough that the module never asks. Command the fans with a scan tool — fans that run on command have a demand problem, and fans that do not have a supply or motor problem.